Computing Mass Properties
In structural mechanics analysis, especially when modeling dynamic problems, the mass properties of a structure or its part can be an important aspect of the design. To compute such mass properties, you can use Mass Properties node, which can be added under Component>Definitions. There, you can select the geometry domains to be included into the computations and select physics interfaces that will define the mass properties. You can add and configure several mass property contributions if needed.
Volume, mass, center of mass, and moments of inertia will be computed. They will become available as predefined variables, which you find in the Equation View node’s settings window under the corresponding Mass Properties node.
Besides for postprocessing purposes, you can also use these variables in any user-defined expressions, user inputs, and in optimization criteria.
Structural mechanics interfaces contribute to the mass properties in several ways:
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All material models, including Rigid Domain, define mass density contributions.
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Added Mass nodes for all geometric entity levels can also contribute with added mass density. It is possible to suppress the contribution from an Added Mass node by using the Exclude contribution check box in the Frame Acceleration Forces section.
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Point Mass nodes contribute both with mass and with the specified mass moment of inertia. It is possible to suppress the contribution from a Point Mass node by using the Exclude contribution check box in the Frame Acceleration Forces section.
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The Hygroscopic Swelling nodes, which can be added to most material models for all structural elements, can use the moisture content as an extra mass density contribution.
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Rigid Domain and Rigid Connector can contribute directly to the total mass properties via their Mass and Moment of Inertia subnodes.
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Structural elements like beams and shells take their true geometrical dimensions into account when contributing to the mass properties. As an example, a beam contributes to the rotational inertia around its axis, even though the geometrical model is only an edge. The beam cross-section properties are used to compute the data.
The mass properties can be computed on both initial geometry (material frame) and deformed geometry (spatial frame). The results may differ considerably in case of large deformations. To compute the results in the undeformed geometry, you do not have to perform the whole analysis, it is sufficient to choose Get Initial Values under the Study node. To obtain the mass properties in the deformed configuration, you need to the full analysis, so that the displacement results are available.
In the COMSOL Multiphysics Reference Manual:
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Mass Properties
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Studies and Solvers
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Derived Values, Evaluation Groups, and Tables